The present invention is directed to a method and apparatus for controlling an electric assist motor. In particular, the present invention is directed to a method and apparatus for controlling an electric motor of an electric assist steering system using a modified blending filter.
Electric assist steering systems are well known in the art. In such electric assist steering systems, an electric assist motor, when energized, provides steering assist torque to aid the driver in turning steerable wheels of the vehicle. The electric assist motor is typically controlled in response to both steering torque applied to the vehicle steering wheel
The present invention is directed to a method and apparatus for controlling an electric assist motor. In particular, the present invention is directed to a method and apparatus for controlling an electric motor of an electric assist steering system using a modified blending filter.
Electric assist steering systems are well known in the art. In such electric assist steering systems, an electric assist motor, when energized, provides steering assist torque to aid the driver in turning steerable wheels of the vehicle. The electric assist motor is typically controlled in response to both steering torque applied to the vehicle steering wheel and measured vehicle speed. A controller monitors steering torque and controls a drive circuit which, in turn, supplies electric current to the electric assist motor. Such drive circuits typically include field effect transistors (xe2x80x9cFETsxe2x80x9d) or other forms of solid state switches operatively coupled between the vehicle battery and the electric assist motor. Motor current is controlled by pulse width modulation (xe2x80x9cPWMxe2x80x9d) of the FETs.
On-center feel is defined as the responsiveness of the steering system for a vehicle traveling in a substantially straight line. Good on-center feel occurs when the driver senses the vehicle lateral acceleration for small steering wheel angle inputs and when the vehicle travels in a straight line with minimal input from the driver. A vehicle that tends to wander or drift from the desired straight line is considered to have poor on-center feel.
Off-center feel is the responsiveness of the steering system in a steady state turn. Good off-center feel occurs when the driver, while in a steady state turn at a high vehicle speed, e.g., on a curved entrance ramp onto a freeway, can easily make small changes in the steering wheel angle that clearly modify the vehicle path. If the angular corrections are difficult to make due to high friction or hysteresis, or if the corrections do not causally modify the vehicle""s path, the vehicle is characterized as having poor off-center feel.
At high vehicle speeds, it is desirable to provide good off-center response as well as good on-center feel. To accomplish this, a trade-off is made in selection of the torque signal to obtain acceptable on-center feel and off-center responsiveness.
Known electric assist steering systems have a dynamic performance characteristic, i.e., system bandwidth, that varies as a function of vehicle speed. As the vehicle operator applies steering torque and rotates the steering wheel back-and-forth, the electric assist motor is energized to provide steering assist in response to the sensed steering inputs. The response of the steering system at a particular frequency of back-and-forth steering wheel movement is indicative of the system""s dynamic performance. The frequency range over which the steering system satisfactorily responds is the system""s bandwidth.
The amount of local change at the electric assist motor divided by the amount of local change in steering torque applied by the driver is the steering system gain. Due to the control function of processing the sensed torque into a desired motor command, a time delay occurs from the time steering torque is applied to the steering wheel to the time the assist motor responds. This time delay is a function of the frequency at which the input command is applied. This is referred to as the system response time. The system gain is set to a predetermined value so as to have a short system response time while still maintaining overall system stability. The system response time and system gain are factors in the steering system bandwidth.
The bandwidth of a steering system varies as a function of vehicle speed. If dynamic steering frequency or the frequency of a transient steering input in an electric assist steering system exceeds the system bandwidth at a particular vehicle speed, the steering feel becomes xe2x80x9csluggishxe2x80x9d (felt as a xe2x80x9chesitationxe2x80x9d to a steering input) since the steering assist motor can not respond quick enough. Steering system gain as well as system bandwidth decreases in an electric assist steering system as the vehicle speed increases resulting in system hesitation or sluggishness becoming more noticeable as vehicle speed increases.
The present invention provides a method and apparatus for improving the steering feel in an electric motor in an electric assist steering system. A high frequency assist gain value is determined in response to vehicle speed and applied steering torque. The high frequency assist gain value is used to control a torque command value so as to provide good off-center tracking as well as good on-center feel.
The present invention is directed to a method for controlling an electric assist motor for providing steering assist in response to a sensed torque signal. The method comprises the step of filtering the sensed torque signal xcfx84s to provide a low frequency torque signal xcfx84sL and a high frequency torque signal xcfx84sH. A low frequency assist torque signal xcfx84assistLF is determined as a function of the low frequency torque signal xcfx84sL. A high frequency assist gain signal Kmax is determined as a function of the sensed torque signal xcfx84s and a sensed vehicle speed xcexd. The high frequency assist gain signal xcexamax is applied to the high frequency torque signal xcfx84sH to determine a high frequency assist torque signal xcfx84assistHF. A torque command signal xcfx84cmd is determined as a function of the low frequency assist torque signal xcfx84assistLF and the high frequency assist signal xcfx84assistHF. The electric assist motor is commanded to provide steering assist in accordance with the torque command signal xcfx84cmd.
The present invention is also directed to an apparatus for controlling a vehicle electric assist steering motor. The apparatus includes a vehicle speed sensor that provides a speed signal having a value indicative of sensed vehicle speed. An applied steering torque sensor provides a sensed torque signal indicative of the applied steering torque. The apparatus also includes filtering means that filters the sensed torque signal to provide a low frequency torque signal and a high frequency torque signal. Means for determining a low frequency assist torque value as a function of the low frequency torque signal provides a low frequency assist torque signal. Means for determining a high frequency assist gain value as a function of the sensed torque signal and a sensed vehicle speed provides a high frequency assist gain signal. The apparatus also includes means for determining a high frequency assist torque value related to the product of the high frequency torque signal and the high frequency assist gain signal and providing a high frequency assist torque signal. Means for determining a torque command value as a function of the low frequency assist torque signal and the high frequency assist torque signal provides a torque command signal. The apparatus further includes motor commanding means that commands the electric assist motor to provide steering assist in accordance with the torque command signal.